The present invention relates generally to the detection and measurement of forces and more particularly to an electrostatically driven vibratory force transducer for an accelerometer.
A widely used technique for force detection and measurement employs a mechanical resonator having a frequency of vibration proportional to the force applied. In one such mechanical resonator, one or more elongate beams are coupled between an instrument frame and a proof mass suspended by a flexure. An electrostatic, electromagnetic or piezoelectric force is applied to the beams to cause them to vibrate transversely at a resonant frequency. The mechanical resonator is designed so that force applied to the proof mass along a fixed axis will cause tension or compression of the beams, which varies the frequency of the vibrating beams. The force applied to the proof mass is quantified by measuring the change in vibration frequency of the beams.
Recently, vibratory force transducers have been fabricated from a body of semiconductor material, such as silicon, by micromachining techniques. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical resonator device, such as a rate gyroscope or an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, xe2x80x9cMethod of Making an Electrostatic Silicon Accelerometerxe2x80x9d and U.S. Pat. No. 4,945,765 xe2x80x9cSilicon Micromachined Accelerometerxe2x80x9d, the complete disclosures of which are incorporated herein by reference.
In electrostatically driven transducers, the elongate beam(s) are typically vibrated by a drive electrode(s) positioned adjacent to or near each beam. A drive voltage, e.g., alternating current, is applied to the drive electrode(s) in conjunction with a bias voltage to generate an electrostatic force that vibrates the beam(s) at a resonant frequency. Motion of the beam(s), in turn, generates a bias current between the electrode and the beam(s) to produce an electrical signal representing the vibration frequency of the beam. Typically, high bias voltages are considered desirable because the current signal from the charging capacitance is proportional to the bias voltage. Therefore, increasing the bias voltage increases the signal to noise ratio of the resonator, and it requires less amplifier gain for the oscillator circuit.
One of the drawbacks with existing electrostatic drives is that the resonance frequency of the vibrating beam is sensitive to the bias voltage. This is because the electrostatic force applied to the beam is inversely proportional to the distance between the drive electrode and the vibrating beam. As the beam moves toward the electrode, the electrostatic force increases, the change in force working opposite to the elastic force of the beam. Likewise, when the beam moves away from the electrode, the electrostatic force which pulls the beam toward its rest position decreases, so that the change in electrostatic force again works against the elastic restoring force of the beam. Thus, the bias voltage acts as an electrical spring that works against the mechanical spring of the system to lower the resonance frequency. Accordingly, electrostatically driven resonators typically require precise regulation of the bias voltage to ensure accurate detection and measurement of the applied force.
Another important consideration in the manufacture of miniature vibratory force sensing resonators is to minimize variations in the frequency signal from the vibrating beams (except for frequency variations responsive to the applied force). To that end, manufacturers of these devices typically strive to maximize the resonance amplification factor (Q) of the vibrating beams, which generally represents the sharpness of the resonances. The resonance amplification factor or Q is typically maximized by partially or completely evacuating the chamber surrounding the transducer to reduce viscous damping of the resonator beams. Thus, vibratory transducers ideally operate in a vacuum to increase the Q and thereby increase the signal-to-noise ratio of the transducer.
On the other hand, it is desirable, and sometimes necessary, to provide viscous gas damping of the proof mass. Gas damping typically involves locating external fluid, such as air, in contact with the proof mass, thereby controlling the effects of a resonance which would deteriorate the performance of the device. For example, this resonance can cause the proof mass to oscillate back and forth about its rest position after a force has been applied to the proof mass. Undesirable resonance can also be caused by vibrations in the surrounding environment that cause the proof mass to oscillate. Unfortunately, gas damping of the proof mass generally requires the gas pressure within the chamber surrounding the transducer to be substantially above vacuum pressure (typically, on the order of about one atmosphere). This gas pressure becomes trapped between the vibrating beams and the electrodes of the transducer, thereby damping the beam""s lateral vibration and reducing their Q. Thus, designers of miniature vibratory transducers are often faced with a compromise between providing gas damping of the proof mass and having a sufficiently high Q to effectively operate the transducer.
What is needed, therefore, are improved methods and apparatus for detecting and measuring forces, such as the force resulting from the acceleration of a proof mass. These methods and apparatus should be capable of operating effectively (i.e., with a sufficient signal-to-noise ratio) at pressure levels above vacuum to permit gas damping of the proof mass. In addition, these methods and apparatus should be designed to minimize the sensitivity of vibration frequency to the drive voltage to increase the accuracy of the force detection device.
The present invention provides methods and apparatus for detecting and measuring forces with mechanical resonators. These methods and apparatus are useful in a variety of applications, such as angular rate sensors, gyroscopes, accelerometers and the like. The methods and apparatus of the present invention are particularly useful for measuring acceleration, such as the acceleration of a miniature proof mass in a micromachined accelerometer.
The apparatus of the present invention includes a vibratory force transducer for an accelerometer having at least one elongate beam with first and second fixed end portions and a resonating portion therebetween. An electrode is positioned adjacent the beam for generating an electrostatic force to transversely vibrate the resonating portion of the beam at a resonant frequency. The resonant frequency of the beam is generally related to the axial (i.e., compressive or tensile) force applied-to between the fixed ends of the beam so that the magnitude of the axial force applied can be measured by changes in the resonant frequency. According to the invention, the electrode and the beam each have a plurality of fingers extending laterally outward so that the beam fingers and the electrode fingers are intermeshed with each other. Applicant has discovered that the intermeshed fingers of the present invention reduce the transducer""s sensitivity to changes in applied voltage, thereby increasing the accuracy of the frequency signal. In addition, the intermeshed fingers of the present invention enable the transducer to operate effectively (i.e., at a Q sufficient to obtain a high signal-to-noise ratio from the vibrating beams) at pressure levels substantially above vacuum, which permits critical gas damping of the proof mass.
In a specific configuration, the transducer of the present invention comprises a pair of substantially parallel beams disposed adjacent to each other and having common ends to form a double ended tuning fork arrangement. The transducer further comprises an elongate electrode located on either side of the beams, and means for generating a drive voltage between each electrode and the adjacent beam. The drive voltage, together with bias voltage from a suitable DC source, produces an electrostatic force that laterally vibrates the beams. Motion of the beams changes the capacitance between the electrode and beam, resulting in a signal which is amplified and used as the drive voltage to drive the beams so that the beams can be used as a resonator in an oscillator circuit. Each beam and electrode have a plurality of intermeshed fingers that are sized and configured so that the distance between the beam fingers and the electrode fingers remains substantially constant as the beams vibrate laterally relative to the electrodes. The electrostatic force between the beams and the electrodes is proportional to the change in capacitance with distance. Since the capacitance between the intermeshed electrode and beam fingers changes linearly with the motion of the beams, the electrostatic force will remain substantially constant as the beams move toward and away from the electrodes. Thus, the resonant frequency of the beams remains substantially constant during beam vibration, which increases the accuracy of the transducer and permits larger drive voltages to be applied to the electrodes, resulting in a larger signal-to-noise ratio and requiring less amplifier gain in the oscillator circuit.
In an exemplary configuration, the accelerometer includes a proof mass suspended from a stationary frame by one or more flexures, and a vibratory force transducer mounted to the proof mass. Preferably, fluid, such as air, is located against the proof mass to damp the proof mass, thereby minimizing proof mass oscillations that may deteriorate the performance of the transducer. To accomplish this, the accelerometer is located within a chamber having an air pressure substantially above vacuum pressure (on the order of about {fraction (1/10)} to 1 atmosphere). Typically, this high pressure environment would reduce the Q of the resonator and decrease the accuracy of the accelerometer. The intermeshed fingers of the beams and electrodes of the present invention, however, minimize gas damping of the beams and, therefore, maintain the Q of the resonator at a sufficient level to provide a good quality acceleration signal at atmospheric pressure. Applicant believes that this occurs because, in the configuration of the present invention, the intermeshed fingers generally do not move toward and away from each other as the beams vibrate, and therefore, the air between these fingers contributes substantially less to damping of the vibrating beams.